The major goal of this proposal is the understanding of the nature of higher order color mechanisms and their possible role in determining the color appearance of objects in complex visual arrays. By higher order mechanisms are meant elements in the chain of visual processing beyond the second stage mechanisms of most conventional models of vision. The existence of such mechanisms has been suggested by the detailed analysis of the selective desensitization resulting from the viewing of modulated fields, and by the relation between the discrimination and detection of pulse changes in color. Experiments are planned using new procedures which are expected to desensitize the higher order mechanisms more strongly than prior stages. In addition, a more detailed examination of the discriminability of pulse changes near detection threshold are planned. New precise and efficient methods of measuring simultaneous color contrast will be used in parametric studies. One goal of these experiments is a general model which will allow prediction of color appearance in complex visual arrays. Another goal is to shed light on the mechanistic levels of the processes of simultaneous contrast. Applications of the new methods of controlling and specifying chromatic stimuli to the measurement of defective color vision are proposed. These methods will not only be capable of efficiently diagnosing congenital color deficiencies but would be applicable to discovery of defects in post-retinal systems. Continued collaboration in studies of the properties of single neurons in the monkey visual system is planned. Methods which have proved fruitful in the characterization of unis in the lateral geniculate will be applied to cortical systems in V1, V2, and V4.